This invention relates to novel epoxy resins and electrostatographic toner containing the same as a binder.
Known developers used in electrostatography are generally classified into one component and two component types. The two component type developer is a mixture of a toner or colored powder and a carrier or chargeable particles. The toner contains a coloring agent such as a pigment and dye, a resinous binder, a lubricant, and other additives.
In electrostatography, toner is fixed onto a substrate, for example, a sheet of paper by three typical methods, a pressure fixing method of passing between two or more metallic rolls to fix the toner to the paper under mechanical pressure, a thermal fixing method of heating the toner to above the melting point thereof to fuse the toner to the paper, and a heat roll fixing method which is a combination of the foregoing two methods.
The heat roll fixing method is improved in fast copying, but suffers from so-called offset that part of the toner adheres and transfers to the heat roll surface because the toner in a molten state is brought in contact with the heat roll. The molten state of the toner causing offset largely depends on the melting point of a binder resin in the toner.
Various resins were proposed in the prior art as the binder resin, including polyethylene, styrene-acryl copolymers, polyesters, and epoxy resins. Styrene resin has been widely utilized among them because it incurs little offset and thus has satisfactory high-temperature fixing properties.
Electrostatography has spread the area of application in these years, stepping out to household appliances. Such application requires a reduction of power consumption. Particularly in color copying, the fixing temperature of toner should be lowered to fix the toner to the whole surface of a sheet. When an electrostatographic equipment is used as a high-speed printing terminal for a computer or the like, high speed fixing is required.
In consideration of a reduction of the power consumed by a heat roll and high-speed copying, the toner and hence, the binder resin is desired to have a lower melting point. A toner having improved fixing properties at low temperatures is available with the use of an epoxy resin as the binder resin because the epoxy resin has a lower melting point than the styrene resin.
The styrene resin has another drawback. Unfortunately styrene is soluble in plasticizers for polyvinyl chloride such as dioctyl phthalate. If a copy is stored in contact with a flexible polyvinyl chloride film, the toner on the copy transfers and adheres to the film to stain the film. The epoxy resin is free from such a phenomenon.
The epoxy resin, however, tends to incur offset because it has a low molecular weight and a low melt viscosity. Particularly, in the case of commonly used bisphenol and hydrogenated bisphenol type epoxy resins, those resins having a lower melting point and hence, a lower molecular weight have an increased tendency to incur offset.